In general, a printed wiring board is fabricated through steps in which an insulating substrate is adhered to a copper foil to make a copper-clad laminate and a conductive pattern is formed on the copper foil surface by etching. High-density implementation of mounted components and handling of signals at higher frequencies have been progressed along with the increase of the recent needs for smaller electronic devices with a higher performance, and printed wiring boards are needed to have a fine conductive pattern (fine pitch) and to deal with high frequencies, for example.
Recently, while a copper foil having a thickness of 9 μm or less, or even a thickness of 5 μm or less has been required to cope with a fine pitch, such an ultrathin copper foil has a low mechanical strength, and is likely to tear or generate a wrinkle in fabricating a printed wiring board. Accordingly, a copper foil provided with a carrier has been developed in which an ultrathin copper layer is electrodeposited above a thick metal foil, which is utilized for a carrier, with a peel layer sandwiched therebetween. The surface of the ultrathin copper layer is pasted on an insulating substrate to heat and pressure-bond, and thereafter the carrier is peeled off and removed via the peel layer. A circuit pattern is formed with a resist on the exposed ultrathin copper layer and then a predetermined circuit is formed.
Here, in order to increase the integrated circuit density of a printed wiring board, a method is common in which a laser hole is formed and the inner layer and the outer layer are connected through the hole. In addition, because a method (MSAP: Modified-Semi-Additive-Process) in which a wiring circuit is formed on an ultrathin copper layer and the ultrathin copper layer is then etching-removed with a sulfuric acid-hydrogen peroxide etchant is employed for a method for forming a fine circuit in association with the popularization of a narrow pitch, the laser hole-opening properties of an ultrathin copper layer are an important matter of concern to produce a high-density integrated circuit substrate. The laser hole-opening properties of an ultrathin copper layer are involved in various conditions such as hole diameter precision and laser output and hence significantly influence the design and productivity of an integrated circuit.
In a common laser hole opening processing, the ultrathin copper layer surface is subjected to a blackening treatment or a fine irregularization treatment with a chemical solution in order to increase the absorbability to a laser wavelength, and thereafter laser hole opening is performed. However, along with the popularization of high integration, it has become common that the ultrathin copper layer surface is directly irradiated with a laser to open a laser hole without being subjected to the above treatments. The commonly used laser is a carbon dioxide laser and copper has a property to reflect the wavelength region of the laser. As a result, the laser hole-opening properties are not improved without performing a treatment such as roughening of the surface. As the technique, Patent Literature 1 discloses that a copper-clad laminate having good laser hole-opening properties can be provided using a waved copper foil for the outer layer copper foil of a copper-clad laminate.